Thermoplastic elastomers show rubber elasticity at room temperature and are easy to mold since they are readily plasticized, or melt, when heated. Also, these materials are recyclable. Because of these advantages, thermoplastic elastomers have recently become widely used in automobile parts, parts for home electric appliances, construction materials, toys, sports equipment, daily necessities, and various other applications. They are also used as stretchable materials in sanitary products, medical materials, belt materials, and other miscellaneous items.
Of different thermoplastic elastomers, polyurethane-based thermoplastic elastomers are most widely used as stretchable materials because of their good extension stress and good stress relaxation property. Styrene-based thermoplastic elastomers, such as styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), and hydrogenated products thereof, are also widely used because of their cost efficiency, flexibility, rubber elasticity, and recyclability.
As for the styrene-based thermoplastic elastomers, attempts have been made to improve different physical properties. The results of such attempts include: <1> a thermoplastic elastomer resin composition for powder molding that contains a styrene-based thermoplastic elastomer, a polyurethane-based thermoplastic elastomer and other components and which can be used to make molded articles that have a soft texture and a high scratch resistance (See, Japanese Patent Laid-Open Publication No. 2001-158812); and <2> a thermoplastic elastomer composition that provides a high flexibility, high formability, and high scratch resistance and that comprises a composition consisting of a styrene-based thermoplastic elastomer (hydrogenated block copolymer) and a methacrylic resin, and a copolymer comprising units that are compatible to the two components of the composition (See, Japanese Patent Laid-Open Publication No. Hei 5-230322).
Also proposed are thermoplastic resin compositions that have flexibility and provide a low temperature performance while preserving favorable properties of acrylic resins, including surface properties such as surface hardness, weather resistance, and clear appearance. Among such thermoplastic resin compositions are <3> an acrylic thermoplastic resin composition containing, at a predetermined ratio, a hydrogenated product of a block copolymer that contains a polymer block (a) composed of an aromatic vinyl compound having a predetermined molecular weight and a polymer block (b) composed either of isoprene and a mixture of isoprene and butadiene; and an acrylic resin having a predetermined intrinsic viscosity (See, Japanese Patent Laid-Open Publication No. Hei 6-329865); and <4> a thermoplastic resin composition containing at a predetermined ratio an acrylic resin and a hydrogenated product of a triblock copolymer that has an a-b-a structure (where “a” is a block composed of an aromatic vinyl compound; and “b” is a block composed of isoprene and/or butadiene) and has a predetermined number average molecular weight (See, Japanese Patent Laid-Open Publication No. Hei 5-295216).
Compositions that can provide a high flexibility and high weather resistance while offering a favorable appearance are also proposed. One example is <5> a thermoplastic resin composition comprising a polyolefin resin (A); a hydrogenated product of a thermoplastic block copolymer (B), composed of a block composed of an aromatic vinyl compound and a block composed of isoprene and/or butadiene; an acrylic resin (C); a hydrocarbon-based softener (D); and a hydrogenated product of a thermoplastic block copolymer (E), composed of a block composed of an aromatic vinyl compound having a side chain of polymerized acrylic monomer and a block composed of isoprene and/or butadiene (See, Japanese Patent Laid-Open Publication No. Hei 5-345841). Also, <6> an adhesive tape is disclosed that has a sufficient tensile strength and a properly low tear strength. This adhesive tape includes a substrate formed of a composition that contains at a predetermined ratio an acrylic polymer composed mainly of a methyl methacrylate; and a hydrogenated product of a block copolymer, including a polymer block composed of an aromatic vinyl compound and a polymer block composed of isoprene and/or butadiene (See, Japanese Patent Laid-Open Publication No. 2000-303037).
The composition <1> described above has a somewhat improved scratch resistance, though not as high as that of polyurethane-based thermoplastic elastomers. Nevertheless, this composition, lacking required hydrolysis resistance and weather resistance, poses problems such as decrease in performance and yellow discoloration when formed into molded articles. Each of the compositions <2> through <5> described above is highly flexible and has high formability and transparency while retaining surface properties, such as surface hardness, weather resistance, and clear appearance, each of which is inherent to acrylic resins. As for the composition <2>, the scratch resistance was evaluated according to JIS Z 8741 in which test samples were rubbed 100 times with a piece of cloth (Kanakin No. 3) while applying a load of 500 g and the glossiness of the sample surface remaining after the test was compared with the initial glossiness of the sample. For the compositions <3> to <5>, the pencil scratch resistance was evaluated according to JIS K 5400. However, none of the compositions <3> to <5> had proven to have sufficient scratch resistance or abrasion resistance. For the composition <6>, nothing is mentioned concerning the abrasion resistance of the composition. Under such circumstances, a need exists for a thermoplastic polymer composition that is suitable for use in applications where it is expected to be subjected to frequent friction or in applications where aesthetic appearance is important.
In regard of the styrene-based thermoplastic elastomers, several proposals have been made with the aim of improving extension characteristics, including formability and extension stress, of stretchable materials formed of the styrene-based thermoplastic elastomers. One example is <7> a stretchable nonwoven fabric described in Japanese Patent Laid-Open Publication No. Hei 3-130448, which shows superior extension characteristics (e.g., stretch and stretch recovery), high strength (e.g., water pressure resistance), and good light resistance and has a soft texture. The stretchable nonwoven fabric is made of a fiber, which comprises a hydrogenated block copolymer obtainable by hydrogenating a block copolymer that includes at least two polymer blocks A composed mainly of an aromatic vinyl compound and at least two polymer blocks B composed mainly of a conjugated diene compound, and a polyolefin. The hydrogenated block copolymer and the polyolefin are mixed at a predetermined weight ratio. The nonwoven fabric is made of ultrafine fiber with an average fiber size of 10 μm or less. Another example is <8> a stretchable nonwoven fabric described in Japanese Patent Laid-Open Publication No. Hei 2-259151, which has superior extension characteristics. The stretchable nonwoven fabric comprises a thermoplastic fiber made of a hydrogenated product of a block copolymer that includes at least two polymer blocks A composed mainly of an aromatic vinyl compound and at least two polymer blocks B composed mainly of a conjugated diene compound with at least one of the polymer blocks B being situated at one end of the polymer chain. The number average molecular weight of the hydrogenated product and the amount of the aromatic vinyl compound are within specific ranges. Still another example is <9> a stretchable nonwoven fabric described in Japanese Patent Laid-Open Publication No. Sho 63-203857 that shows a good extension recovery, flexibility, and light resistance. This stretchable nonwoven fabric is made of a thermoplastic polymer composition that contains a thermoplastic polymer (a) having a polar functional group, such as polyamide and polyester; and a modified block copolymer and/or a modified block-graft copolymer (b) in which a block copolymer consisting of an aromatic vinyl compound polymer block and a conjugated diene compound polymer block and/or a hydrogenated product thereof, or a block-graft copolymer in which the copolymer serves as the backbone and a radically decaying polymer serves as the grafts, are bound to a molecular unit having a functional group capable of binging to, or interacting with, the thermoplastic polymer (a) (e.g., maleic anhydride group).
While polyurethane-based thermoplastic elastomers are favorable in terms of formability, stress relaxation property, and extension stress, their hydrolysis resistance and weather resistance are poor, often resulting in a significant reduction in performance or yellow discoloration. Also, each of the stretchable materials <7> to <9> described above is not satisfactory as far as the balance among formability, stress relaxation property, tensile permanent set, and extension stress is concerned.
To impart additional functions to materials such as resin to be used as a substrate, a plurality of materials are laminated on top of one another. Such laminates are used in a variety of fields, including automobile parts, parts for home electric appliances, construction materials, furniture, toys, sports equipment, and daily necessaries.
Soft vinyl chloride resins are inexpensive materials and have superior surface characteristics, such as scratch resistance and abrasion resistance, as well as flexibility. For this reason, these resins are preferred materials for use in the above-described laminates, especially in the outermost layer of the laminates. Nevertheless, soft vinyl chloride resins have several drawbacks: plasticizers contained in the resins seep out; such plasticizers are potential endocrine disruptors; and the resins generate corrosive gases, such as hydrogen chloride, and highly toxic dioxins when incinerated.
Olefin-based thermoplastic elastomers, styrene-based thermoplastic elastomers, and polyurethane-based thermoplastic elastomers each have superior surface characteristics, such as high scratch resistance and high abrasion resistance, as well as flexibility, and can thus be used to form laminates. In particular, olefin-based thermoplastic elastomers, which are favorable in terms of strength and cost, and styrene-based thermoplastic elastomers, which are favorable in terms of mechanical properties and flexibility, do not pose the problems associated with the soft vinyl chloride resins and are considered as an alternative to the soft vinyl chloride resin. Laminates using these materials have been proposed (See, for example, Japanese Patent Laid-Open Publications No. Hei 4-73112, No. Hei 4-73142, and No. Hei 8-90723). Also, <10> a method is described by which a surface layer composed mainly of a polyurethane-based thermoplastic elastomer, an adhesive resin layer, and a substrate layer or a foam layer composed mainly of a thermoplastic resin are coextruded to form a sheet-like laminate (See, for example, Japanese Patent Laid-Open Publications No. Hei 7-68623 and No Hei 7-290625). <11> A laminate described in Japanese Patent Laid-Open Publication No. Hei 6-8381 comprises (i) a thermoplastic resin layer; and (ii) a layer formed of a composition obtained by adding a predetermined amount of a hydrogenated product of a thermoplastic block copolymer, the thermoplastic block copolymer having a polymerized acrylic monomer side chain and consisting of a block composed of an aromatic vinyl compound and a block composed of isoprene and/or butadiene, to 100 parts by mass of a predetermined mixture of an acrylic resin and a hydrogenated product of a thermoplastic block copolymer composed of a block composed of an aromatic vinyl compound and a block composed of isoprene and/or butadiene.
However, nothing is mentioned about the scratch resistance or the abrasion resistance of the outermost layers of the laminates in any of Japanese Patent Laid-Open Publications No. Hei 4-73112, No. Hei 4-73142, and No. Hei 8-90723. In regard of the laminate <10> described above, when olefin-based materials are used in the substrate layer of the laminate, an adhesive resin layer must be provided to adhere the substrate layer. This adds to the complexity of the production process. The laminate <10> also has a problem that the insufficient hydrolysis resistance and the insufficient weather resistance of the polyurethane-based thermoplastic elastomer may cause a reduction in the performance. While the laminate <11> as described above is favorable in terms of flexibility, weather resistance, appearance-related properties, and adhesion, nothing is mentioned about the scratch resistance or the abrasion resistance of the laminate.
Various foams that make use of styrene-based thermoplastic elastomers have been proposed for the purposes of reducing weight and providing cushioning property. Among such foams are <12> an extruded foam article using a composition composed of a styrene-based thermoplastic elastomer having a predetermined melt tension, melt malleability, hardness (JIS-A), and melt flow rate (MFR), and a blowing agent (See, Japanese Patent Laid-Open Publication No. Hei 7-18106); <13> a expandable thermoplastic elastomer composition, comprising a block copolymer composed of at least two polymer blocks A formed mainly of an aromatic vinyl compound and at least one polymer block B formed mainly of a conjugated diene, and/or a hydrogenated product thereof; a peroxide-degradable olefin-based resin and/or a copolymer rubber containing the same; a polyethylene-based resin polymerized by a single-site catalyst; a softener for non-aromatic rubbers; and a heat-expandable microcapsule that expands at a temperature of 100° C. to 200° C. (See, Japanese Patent Laid-Open Publication No. 2000-17140); and <14> a thermoplastic polymer foam, obtainable by foaming a thermoplastic polymer composition containing a thermoplastic acrylic polymer and a hydrogenated block copolymer at a weight ratio of 80:20 to 20:80, the hydrogenated block copolymer containing a polymer block composed of an aromatic vinyl compound and a polymer block composed of a conjugated diene (See, Japanese Patent Laid-Open Publication No. Hei 9-241414).
The foam article <12> has a superior molded appearance, flexibility, and low-temperature impact resistance, and the expansion ratio of the foam article is high. The expandable thermoplastic elastomer composition <13> can form articles with a favorable appearance and texture even when the expansion ratio is considerably high. Finally, the thermoplastic polymer foam <14> is highly flexible and retains the flexibility at low temperatures. Since this material does not contain any plasticizers, it does not pose problems such as the plasticizer seeping out or being transferred. Nonetheless, nothing is mentioned concerning the scratch resistance or the abrasion resistance for any of the foams <12> to <14>, nor can any teaching be found in the respective publications regarding the production of foams that have a good heat resistance, in particular, regarding the production of those that exhibit a superior compression permanent set at high temperature (e.g., at 70° C.). Thus, a need exists for a foam that is suitable for use in applications where it is expected to be subjected to frequent friction or high temperature conditions.
Accordingly, it is an object of the present invention to provide a polymer composition that has a high formability, flexibility, rubber elasticity, mechanical strength, and transparency while exhibiting a scratch resistance and abrasion resistance comparable to those of polyurethane-based thermoplastic elastomers and polyester-based thermoplastic elastomers. By exploiting these characteristics, such polymer compositions can be effectively used in a wide range of applications, including stretchable materials, laminates, and foams.
It is another object of the present invention to provide a stretchable material that does not pose any of the above-identified problems associated with conventional stretchable materials and is formed of a polymer composition that provides a good flexibility, rubber elasticity, mechanical strength, and stress relaxation property while exhibiting a minimum tensile permanent set and being readily formed. This stretchable material exhibits superior extension characteristics, including extension stress.
It is a further object of the present invention to provide a laminate comprising a layer formed of a polymer composition that has a high scratch resistance, abrasion resistance, and flexibility and can readily be manufactured without requiring complicated processes.
It is a still further object of the present invention to provide a foam that not only shows a high heat resistance, in particular high compression permanent set at a high temperature (e.g., at 70° C.), but also an scratch resistance and abrasion resistance comparable to those of polyurethane-based thermoplastic elastomers, while preserving characteristics of styrene-based thermoplastic elastomers, including flexibility and formability. It is also an object of the present invention to provide a foam composition for forming such a foam.
In an effort to find ways to attain the above-described objects, the present inventors have found that by providing a specific polymer composition that contains a block copolymer having a molecular weight in a predetermined range and containing a block composed mainly of α-methylstyrene as its hard segment, an acrylic resin, and an optional softener, and by adjusting the ratio of these components of the polymer composition, the block copolymer containing a block composed mainly of α-methylstyrene as its hard segment forms a continuous phase (i.e., matrix) and the acrylic resin disperses throughout the block copolymer, forming a specific sea-island morphology.
The present inventors examined the physical properties both of this polymer composition having the above-described specific morphology and of molded articles formed of the polymer composition and have found that such polymer compositions show a high formability, exhibit many other favorable characteristics, including flexibility, rubber elasticity, mechanical strength, transparency, and, above all, scratch resistance and abrasion resistance, exhibit these properties in a well-balanced manner, and are therefore suitable for use in a wide range of applications.
For example, the present inventors have found that a stretchable material formed of the polymer composition shows a good stress relaxation property, a small tensile permanent set, as well as superior extension characteristics, including extension stress, and can thus be effectively used in various fields including hygiene products, medical materials and other miscellaneous goods.
The present inventors have also found that a laminate comprising a layer formed of the polymer composition and a layer formed of other materials, preferably, a layer formed of a thermoplastic resin such as an olefin-based resin, olefin-based thermoplastic elastomer, styrene-based thermoplastic elastomer, or a resin composition containing styrene-based thermoplastic elastomer, shows a high scratch resistance, abrasion resistance, and flexibility and can be effectively used in a wide range of applications by exploiting these characteristics.
The present inventors have further found that by adding a predetermined amount of a blowing agent to the polymer composition so as to reduce weight and provide an cushioning property, a foam composition can be obtained that can be foamed into a foam with a good foamability and heat resistance, in particular, a good compression permanent set at a high temperature (e.g., at 70° C.) and that such a foam can be effectively used in a wide range of applications. These findings led the present inventors to devise the present invention.